Volatile organic chemicals are commonly released to the environment as nonaqueous phase liquids (NALS) via surface spillage, storage tank leakage, or leaching from hazardous waste landfills. The presence and behavior of NAPL in subsurface systems has not been adequately accounted for in prior aquifer modeling efforts. The is one of two companion projects to be conducted at the University of Michigan to address that deficiency. This project will focus on the development and partial verification of a computer model capable of quantifying the subsurface movement of NAPL itself, as a separate phase, as well as the movement of gas phases which result from volatilization of NAPL. Both non-aqueous bulk transport and vapor phase transport by convection and diffusion will be modeled. The model will be developed for computational efficiency and flexibility. Extensions of the model to two-dimensional domains and multi-component organic mixtures are also included in this research plan. To verify the physical assumptions inherent in the modeling approach, the behavior of two representative volatile organic compounds will be explored. Constitutive parameters for these two compounds will be measured and column experiments will be conducted for comparison with one-dimensional model simulations. In the later stages of the research, two-dimensional laboratory studies will be conducted to verify the two- dimensional model and explore the use of active and passive gas sampling techniques. The validity of the model assumptions concerning interphase mass exchange will be explored in the companion research project, and the results from that companion project will be incorporated into the overall computer simulator developed in this project.